1. Field of the Invention
The present invention generally relates to a system and method for continuously monitoring and controlling concentration of molecules or chemical species. More specifically, the present invention relates to a system and method for monitoring concentration of fluorescent tracer molecules in industrial water systems. Further, the present invention relates to use of an all solid-state diode-laser or light emitting diode-based fluorometer for monitoring the concentration of fluorescent tracer molecules in aqueous, non-aqueous, and mixed aqueous/non-aqueous systems.
2. Description of the Prior Art
It is generally known to use diode lasers or light-emitting diodes (LED) as solid-state excitation sources for fluorescence. The combination, however, of excitation sources with photodiode detectors is not as common. As early as 1988, a fluorometer from an LED and a photodiode detector was constructed. See, for example, an article by Jones et al. entitled "High Precision Fluorimetry with a Light-Emitting Diode Source," Appl. Spectroscopy 42, 1469 (1988). In 1989, a 670 nanometer diode laser was used as an excitation source and a pholomultiplier (PMT) as a detector. See Imasaka et al. "Visible Semiconductor Laser Fluorometry," Anal. Chem. 61, 2285 (1989). Other examples are known in which semiconductor lasers have been combined with conventional PMT detectors. See, for example, Patonay et al. "Semiconductor Lasers in Analytical Chemistry," Proceedings of SPIE-The International Society for Optical Engineering 1435, 42 (1991); Higashijima et al. "Determination of Amino Acid By Capillary Zone Electrophoresis Based on Semiconductor Laser Fluorescence Detection," Anal. Chem. 64, 711 (1992); and Mank et al. "Visible Diode Laser Induced Fluorescence Detection in Liquid Chromatography after Precolumn Derivatization of Thiols," Anal Chem. 65, 2197 (1993).
In addition, several more recent publications have dealt with fluorescence measurements using LEDs or diode lasers as excitation sources and silicon photodiodes as detectors. See, for example, Hauser et al., "A Solid-State Instrument for Fluorescence Chemical Sensors Using a Blue Light Emitting Diode of High Intensity," Meas. Sci. Technol. 6, 1081 (1995); Wengatz et al., "Immunoassays for Pesticide Monitoring," Proceedings of SPIE-The International Society for Optical Engineering 2388, 408 (1995); Williams et al., "Instrument to Detect Near-Infra-Red Fluorescence in Solid-Phase Immunoassay," Anal Chem. 66, 3102 (1994); and Kawazumi et al., "Laser Fluorimetry Using A Visible Semiconductor Laser and an Avalanche Photodiode for Capillary Electrophoresis," Anal. Sci. 11, 587 (1995).
Of the above, most of the few known literature references demonstrate the principle of fluorometry using solid-state, low cost excitation sources. Only a few of the existing papers, however, deal with applications of this instrumentation. For example, Higashijima et al. generally disclose the use of fluorescence detectors for electrophoresis; Mank et al. generally disclose the use of fluorescence detectors for liquid chromatography; and Hauser et al. relate to use of fluorescence detectors for chemical-sensing membranes. In addition, Wengatz et al. explore the use of fluorescence detectors for pesticide monitoring.
A number of other techniques are known for monitoring fluorescence, for example, from oil residues on steel sheets (such as taught by Montan et al. in "A System for Industrial Surface Monitoring Utilizing Laser-Induced Fluorescence," Appl. Phys. B38, 241 (1985)) and for fluorescence analysis of biologically important molecules in turbid or opaque tissue samples (for example, as demonstrated by Winkleman et al. in "Quantitative Fluorescence Analysis in Opaque Suspensions Using Front Face Optics," Anal. Chem. 39, 1007 (1967)). Furthermore, use of an excimer laser to perform fluorescent imaging of paper surfaces is generally taught by Hakkanen et al. in "Laser-Induced Fluorescence Imaging of Paper Surfaces," Appl. Spectroscopy 47, 2122 (1993); and use of a diode laser in surface fluorescence geometry is also generally taught, for example, by German Patent No. DE4300723 A1.
Fluorometers currently being used for industrial process monitoring and control are based on gas-lamp excitation sources and photomultiplier tube detectors which require high current, high voltage power supplies. Additionally, these excitation and detection sources do not have the intrinsic reliability of solid-state semiconductor devices.
A need, therefore, exists for an improved instrument constructed as an all solid-state fluorometer including a system and method for the use of such a fluorometer for monitoring the concentration of fluorescent tracer molecules particularly in industrial water systems.